Current directory (lodaer.path=.) pathology workaround

It turns out that loader.path=. was pathological and before this
change ended up making the classpath empty (loader.path=.,lib/
would have fixed it). With this change the old behaviour is still
supported, but if the only user-supplied path entry is "." (or
empty) then it is now kept, and translates into the root of the
current archive if running as "java -jar ...".

Fixes gh-270

spring-boot-cli/src/it/java/org/springframework/boot/cli/JarCommandIT.java

@@ -64,7 +64,7 @@ public class JarCommandIT {
 		Invocation invocation = this.cli.invoke("jar", jar.getAbsolutePath(),
 				"jar.groovy");
 		invocation.await();
-		assertEquals(0, invocation.getErrorOutput().length());
+		assertEquals(invocation.getErrorOutput(), 0, invocation.getErrorOutput().length());
 		assertTrue(jar.exists());
 
 		Process process = new JavaExecutable().processBuilder("-jar",